Ciguatera fish poisoning (CFP), the most common form of finfish-borne disease globally, affecting tens of thousands of people each year, is caused by dinoflagellates in the genus Gambierdiscus. Our goal is to investigate how climate change, particularly warming sea surface temperatures, will influence the geographic extent and population dynamics of toxin-producing Gambierdiscus spp. in the Greater Caribbean Region (GCR), and use these findings to model ciguatoxin fluxes into coral reef food webs under predicted climate change scenarios. This proposal builds on several important findings from our previous work, which includes the determination that toxicity in the GCR is tightly linked to just a few Gambierdiscus species, and that the most toxic species G. silvae may be the ?super bug? on Caribbean reefs, responsible for the bulk of toxin entering the food web even at low cell densities. This breakthrough discovery presents a fortuitous opportunity to study the physiology, toxicity, and ecology of this highly toxigenic species, including its temperature tolerances and the manner in which it disperses to and colonizes other regions. The scientific premise of this project is that only through a deep understanding of the autecology of the most toxic species and their direct and indirect responses to climate change (e.g., warming sea surface temperatures and coral reef impacts, respectively), will we (society) be able to properly assess and respond to the impact of climate change on CFP incidence in the GCR. We will use innovative experimental approaches to examine the impacts of temperature, including field investigations of local adaptation within and dispersal between Gambierdiscus sub-populations using RADseq (Restriction site-Associated DNA sequencing) and analyses of the spatio-temporal dynamics of toxin-producing Gambierdiscus spp. In addition to investigating broad-scale geographic patterns, we will use natural temperature differences between closely adjacent sites to explore the effects of temperature extremes and variability on the composition of Gambierdiscus communities. These natural experiments will be complemented by laboratory studies to determine resilience of toxin-producing Gambierdiscus spp. to thermal stress under variable temperature regimes. Together, these data will reveal how temperature-driven partitioning of Gambierdiscus communities operates at local, regional, and seasonal scales. This has major implications with respect to the effects of climate-driven warming on the extent and prevalence of ciguatera toxicity in the GCR. The resulting data, along with that produced in RP2 and RP3, will enable us to further develop and calibrate our ciguatoxin flux model, which is the first-ever computational model of ciguatoxin fluxes in the food web. This modeling effort will push forward the boundaries of predictive capacity for CFP events and public health protection.